प्रदीपिकोऽग्निना वध्यः ॥ कZ_०२.३६.२५ ॥
pradīpiko 'gninā vadhyaḥ
A person who sets (anything) on fire—an arsonist—is to be put to death by fire.
To create maximum deterrence for a high-externality crime that can destroy dense urban property, granaries, and lives; the punishment mirrors the harm (lex talionis-style proportionality).
Primarily public-order: arson threatens collective security and state assets, justifying the severest penalty.